In modern integrated circuit (IC) fabrication, layers of material are applied to embedded structures previously formed on semiconductor wafers. Chemical Mechanical Planarization (CMP) is an abrasive process used to remove these layers (or portions thereof) and polish the resulting surface to achieve a desired structure. CMP may be performed on both oxides and metals and generally involves the use of chemical slurries applied in conjunction with a polishing pad in motion relative to the wafer (e.g., the pad rotates relative to the wafer with the slurry dispersed therebetween). The resulting smooth flat surface is necessary to maintain photolithographic depth of focus for subsequent wafer processing steps and to ensure that the metal interconnects are not deformed over contour steps. Damascene processing requires metal, such as tungsten or copper, to be removed from the top surface of a dielectric to define interconnect structures, using CMP.
Conventional polishing pads are made of polymers, typically urethane, either in cast form and filled with micro-porous elements or from non-woven felt coated with polyurethanes. During polishing operations, the polishing pad is rotated while contacting the wafer, which is also rotating, thus effecting polishing. Many pads have structures to provide for distributing slurry under the wafer during these polishing processes. Such slurry distribution structures include voids or micro-pores that are included by adding hollow micro-elements as described in U.S. Pat. No. 5,578,362 or through the introduction of bubbles formed during a casting process. U.S. Pat. No. 6,896,593 describes the use of supercritical CO2 to form pores during molding processes.
During polishing, the polishing pad material undergoes plastic deformation that leads to deterioration of the pad's polishing ability. For example, such deformation during a polishing process results in that process becoming non-uniform and, as a result, the polishing operation can be accomplished only at a much lower removal rate.
In order to restore polishing performance and achieve consistent polishing performance, the polishing pad surface is periodically abraded (or conditioned) with a disk covered with fine grit diamond particles. The purpose of such conditioning is to remove the worn top layer of the pad and restore the texture of the top surface so as to effect consistent polishing. Such conditioning processes are well accepted for polishing processes employing hard pads. In the case of soft pads, however, conditioning processes are not as well accepted and, as such, tend to result in reduced useful lifetimes of such pads.